Since silicon carbide (hereinafter referred to as “SiC”) has a wide bandgap and high dielectric breakdown characteristics as compared with other semiconductor materials, applications in low-loss power devices are being anticipated. On an SiC layer, a relatively high quality silicon dioxide (SiO2) film is formed by thermal oxidation of the SiC. It is therefore presumably effective to adopt an insulated-gate structure for an SiC power conversion device.
Nevertheless, a thermal oxide film, as a gate insulating film, formed on an SiC layer has many problems remaining to be overcome. For example, defects in a thermal oxide film cause high-density interface traps to be present in a region of the SiC layer in the vicinity of the interface with the insulating film, which have a profound effect on the electron-transport mechanism in the MIS channel. Specifically, the interface traps existing in the region of the SiC layer in the vicinity of the interface with the insulating film cause significantly reduced channel conductance in the insulated-gate SiC device. However, the mechanism in which defects in a thermal oxide film cause interface traps to form in a region of the SiC layer in the vicinity of the thermal oxide film has hardly been studied so far.
In particular, in an interface of SiO2/SiC in which 4H—SiC is used, acceptor traps, called E′1 centers, are created, due to defects in the oxide film, in the forbidden-band location having potential in the vicinity of the conduction band, and the acceptor traps significantly affect transportation of electrons. 4H—SiC, having a wider bandgap, a higher dielectric breakdown voltage, and a higher bulk mobility than other poly-type crystals such as a 6H—SiC crystal, is a poly type of crystal best suited to be applied in power devices. Due to the above-mentioned acceptor traps, however, a MISFET including an SIC layer of a 4H—SiC crystal has an extremely low channel mobility, which is a major obstacle in practical applications of the SiC device.
As mentioned above, since in inversion MISFETs in which 4H—SiC is used, interface traps affect the electron transport mechanism seriously, various studies have been made in order to increase the channel mobility of the MISFETs. Particularly, in research on accumulation channel MISFETs, raising channel mobility up to twice that of inversion MISFETs has been successfully achieved. In this case, the essential significance of the accumulation channel MISFETs is in their structure in which, in the depth-direction density distribution of current flowing in the channel, the ratio of the current flowing in the deeper portion of the channel region to the current flowing in the surface portion thereof, is increased as compared with the ratio in the inversion MISFETs.